The present invention relates to a steering control system for steering a wheeled vehicle so as to improve a steering characteristic, and a directional stability of the vehicle.
A turning behavior and steering stability of a wheeled vehicle are usually described by a relationship between a steering input, and a yaw rate or a lateral acceleration of the vehicle. The steering input is a driver's command to steer the vehicle. In general, a steering angle .theta. of a steering wheel is regarded as the steering input. The yaw rate (or yaw angular velocity) is an angular velocity of a rotation of the vehicle about a vertical axis passing through the center of gravity of the vehicle. The lateral acceleration is an acceleration of the center of gravity of the vehicle in a lateral direction of the vehicle.
It is desired that a vehicle should be turned to an amount corresponding to the driver's steering input without being affected by a disturbance such as a side wind and a friction coefficient of a road surface. The amount of a turn of a vehicle can be described in terms of a ratio of the yaw rate .phi. (or the lateral acceleration .alpha.) to the steering input (the steering angle .theta. of the steering wheel). This ratio is referred to as a gain of the vehicle, or a yaw rate gain of the vehicle.
In a vehicle having no auxiliary steering control, the gain such as yaw rate gain is enhanced at a certain steering frequency (a resonance frequency H1), as shown by a curve "a" in FIG. 19, so that the behavior of the vehicle responsive to the steering input is changed abruptly at or near this steering frequency. It is desired that, over the full range of the steering frequency, the gain of the vehicle remains at a value obtained when the steering frequency is approximately zero.
Therefore, there have been proposed steering control systems so designed as to make the resonance frequency H1 high and thereby widen the range in which the gain characteristic is flat. One example is shown in FIG. 1. In this system, the steering input is transmitted to the front wheels through the steering gear to steer the vehicle. At the same time, the steering input is sensed by a steering sensor, and differentiated by a differentiator. The derivative determined by the differentiator is sent through an amplifier to an actuator, which steers the front wheels in such a positive direction as to increase the direction change of the vehicle in accordance with the derivative of the steering input. Thus, this system is a feedforward control type. A curve "c" of FIG. 19 shows the gain characteristic obtained by this system. As shown by the curve "b", this system can increase the resonance frequency to H2.
Another example is shown in FIG. 2. This system is a negative feedback type. In this system, the steering input is transmitted through the steering gear to the front wheels to steer the vehicle. At the same time, a vehicle behavior sensor senses a turning behavior of the vehicle, such as the yaw rate or the lateral acceleration of the vehicle. The sensed behavior is multiplied by a feedback coefficient, and sent to an actuator through an amplifier. In accordance with the sensed behavior multiplied by the feedback coefficient, the actuator steers the front wheels in such a negative direction as to reduce the direction change of the vehicle. A curve "c" of FIG. 19 shows the gain characteristic obtained by this system. As shown, this sytem can increase the resonance frequenc to H3.
However, the system of FIG. 1 cannot decrease the gain of the vehicle, and cannot eliminate adverse influence exerted on the turning behavior of the vehicle by a disturbance such as a side wind. The system of FIG. 2 tends to decreas the gain of the vehicle so much that the response characteristic of the turning behavior of the vehicle is made worse.